As known, a positive displacement pump is a particular type of pump that exploits the change in volume in a chamber to cause either a suction or a thrust on an incompressible fluid. Positive displacement pumps include rotary pumps of the gear type, in which the change in volume of the work chamber is obtained through the rotation of elements, typically two toothed wheels that engage with one another, capable of delimiting rotary chambers having variable volume. Gear pumps are widely used in the field of lubrication and, in general, in all applications in which the liquid to be transferred is particularly viscous.
For example, so-called inner gear pumps are built with the two gears arranged one inside the other but on offset axes. A separation assembly takes care of separating the two gears by means of a half-moon shaped dividing wall. The depression caused by the movement of the gears, when the respective teeth move apart, allows liquid to enter into the cavity that is created between the teeth of the gears themselves. When, on the other hand, the teeth of the gears approach one another, an overpressure is created that pushes the liquid towards the discharge area of the pump.
In gear pumps, the transmission of power, generated normally by an electric motor, can take place through so-called “magnetic drive”. This transmission system is provided with two coaxial magnetic rings or cores, mounted one on the drive shaft and the other on the shaft of the impeller, in other words one of the gears of the pump. By applying a torque, the magnetic fields of the core mounted on the drive shaft move towards those of equal polarity of the core mounted on the shaft of the impeller and, through the effect of magnetic repulsion, push it into rotation.
Currently, the components and the power transmission systems of the most common gear pumps are enclosed by sealed containment vessels made of metallic material, typically stainless steel. A cost-effective solution for the packing of these components and the closure of the pump consists of bending the plate of a containment shield on the body of the pump, for example through cold deformation (vertical pressing or lateral rolling).
If the pump is operating at particularly low temperatures and if it is subjected to more or less long periods of inactivity, it is possible for there to be increases in volume of the liquid to be pumped due to the freezing of the liquid itself. The fact that it is impossible for the sealed containment vessel of the pump to compensate for such increases in volume may therefore cause damage to the internal mechanisms of the pump itself.
Document EP 2273121 A2, filed to the same Applicant, describes a containment assembly for a pump configured to compensate for possible increases in volume of the liquid contained inside the pump itself. However, as well as these increases in volume, during the normal operation of the pump excessive tolerances or “clearances” can also be generated between the moving components of the pump itself. These clearances are due mainly to thermal dilations of the components of the pump that occur in opposite work conditions to those mentioned above, in other words in the case of high temperatures. Irrespective of the causes, these clearances can in any case compromise the correct operation of the pump.